The present invention generally relates to gradation (or density) control devices for thermal ink-transfer type printing apparatuses (hereinafter simply referred to as thermal printers), and more particularly to a gradation control device which controls the size of printing dots by controlling an applying time of a constant current which is applied to heating elements of a thermal printing head in order to control the gradation level or printing density in a thermal printer.
Among terminal printers or hard-copy apparatuses such as wire-dot type and ink-jet type printers, thermal printers are being developed as one of the more promising type and are used in copying machines, facsimile machines and the like. For example, the thermal printer employs an ink film which is a polyester film having a thickness of 5 to 6 microns coated with a kind of ink which melts due to heat on one surface thereof. The ink film is placed onto a recording sheet with the ink side making contact with the recording sheet, and a thermal printing head makes contact with a rear side of the ink film. When a current flows through the thermal printing head so as to generate heat at the thermal printing head, the ink on the ink film melts at the position corresponding to the position of the thermal printing head, and the melted ink is transferred onto the recording sheet. The thermal printing head comprises a plurality of heating elements arranged in a row, and a current is applied to each of these heating elements which are to be heated.
The density which determines the gradation level of the printed characters, diagrams, pictures and the like, is determined by the area of each dot formed on the recording sheet. And, this area of the melted ink dot is determined according to the current applied to each of the heating elements. Generally, the heat value becomes larger as the magnitude of the current applied to the heating element becomes larger. As a result, the area of the melted ink dot becomes larger to increase the printing density, and the gradation level reaches near a saturated density. Accordingly, the magnitudes of the currents applied to the heating elements are conventionally controlled in order to control the gradation level of the printing. However, the currents applied to the heating elements ae generally large currents in the order of 5 to 20 Amperes. Thus, it is difficult to control such large currents with a quick response speed, and there are disadvantages in that the size of the gradation control device becomes large and the gradation control device becomes expensive. Furthermore, it is impossible to increase the response speed when controlling such large currents, and there is a disadvantage in that the printing speed cannot be increased.
Accordingly, an improved tone (gradation) control device for a thermal printer was previously proposed in a U.S. Pat. No. 4,532,523 in which the assignee is the same as the assignee of the present application. This previously proposed tone control device controls the printing density by controlling the size of the printing dots according to applying times of currents which are applied to the heating elements of the thermal printing head.
However, even in the case of the previously proposed tone control device, a large current is applied to those heating elements which are to transfer the ink on the ink film onto the recording sheet. For this reason, a voltage drop occurs between a power source and the heating elements of the thermal printing head. The number of heating elements to which the current is applied depends on the printing data, and the voltage drop accordingly changes depending on the printing data. As a result, the current which should be kept constant changes, and there is a problem in that the size of the printing dots actually printed on the recording sheet changes depending on the printing data even when the gradation level is the same. Similar problems occur when a power source voltage from the power source is unstable.
In order to overcome the problem of the voltage drop, it is necessary to use a power source having a large capacity, but it is impractical in that such a power source is bulky and expensive. It is possible to conceive a method of dividing the heating elements of the thermal printing head into a number of heating element groups, and drive the heating element groups time-divisionally so as to reduce the current which is required at one time and enable the use of a compact power source having a smaller capacity. However, this method is impractical in that a printing error will be generated due to time delays in the timings with which the heating element groups are actually driven.
On the other hand, as a measure against the voltage drop between the power source and the heating elements of the thermal printing head, the power source voltage may be corrected by remote sensing, for example. But the correction of the power source voltage by the remote sensing is insufficient in that the response speed is too slow in the case where the heating elements of the thermal printing head are driven by pulse currents.